Glass Transition Onset Temperature Research Articles

Cross-Linking and Evaluation of the Thermo-Mechanical Behavior of Epoxy Based Poly(ionic Liquid) Thermosets.

Poly(ionic liquids) (PILs) and ionenes are polymers containing ionic groups in their repeating units. The unique properties of these polymers render them as interesting candidates for a variety of applications, such as gas separation membranes and polyelectrolytes. Due to the vast number of possible structures, numerous synthesis protocols to produce monomers with different functional groups for task-specific PILs are reported in literature. A difunctional epoxy-IL resin was synthesized and cured with multifunctional amine and anhydride hardeners and the thermal and thermomechanical properties of the networks were assessed via differential scanning calorimetry and dynamic mechanical analysis. By the selection of suitable hardeners, the glass transition onset temperature (Tg,onset) of the resulting networks was varied between 18 °C and 99 °C. Copolymerization of epoxy-IL with diglycidyl ether of bisphenol A (DGEBA) led to a further increase of the Tg,onset. The results demonstrate the potential of epoxy chemistry for tailorable PIL networks, where the hardener takes the place of the ligands without requiring an additional synthesis step and can be chosen from a broad range of commercially available compounds.

Predictions of glass transition onset temperature of chalcogenide glass GexSe1−x

The chalcogenide glass, GexSe1−x, has been widely used in advanced electronics and photonics. The onset temperature of glass transition, Tg, plays an important role in manufacturing processes and practical applications. To investigate the Tg, experimental methods to observe the glass transition step are commonly used, but the correlation between influencing factors and the Tg remains vague. In this study, we develop the multivariate linear regression (MLR) model to present the relationship between physical attributes and GexSe1−xTg. This simple model is highly accurate and contributes to rapid calculations of Tg. Compared with previous models that are more complicated in model construction and applications, the MLR model shows better performance and the direct statistical correlation between descriptors and the target variable.

Predicting As$$_{x}$$Se$$_{1-x}$$ Glass Transition Onset Temperature

The As–Se glass is a large subgroup in the class of chalcogenide glass, which is widely used in electronics and photonics. Glass transition onset temperature, $$T_{g}$$ , is an important thermal parameter that needs to be considered during manufacturing and practical applications. Numerous experimental and theoretical approaches have been conducted to investigate $$T_{g}$$ , but they tend to be resource-intensive and complicated. In this study, we develop the multivariate linear regression model to shed light on the relationship between physical attributes and As $$_{x}$$ Se $$_{1-x}$$ $$T_{g}$$ . The model is simple and highly accurate that contributes to fast estimations of $$T_{g}$$ .

Analysis of Physical and Structural Properties of Alkali Oxide–Modified Tellurite Glasses

Glasses in the system x M2O·(1 − x) TeO2 (0 ≤ x ≤ 0.25), where M = Li, Na, K, Rb, Cs, were made by melting in platinum crucibles. Various quenching methods were used to make each glass because some glasses require extremely rapid cooling. Physical properties such as the glass transition onset temperature (Tg) and density (ρ) were measured. In order to explore the short-range structure in these glasses, Raman spectroscopy, infrared spectroscopy, and high-energy x-ray diffraction were used. These experiments on tellurite glasses were performed to find correlations between the physical and structural properties.

MD study on topologically close-packed and configuration entropy of Mg40Al60 metallic glasses under rapid solidification

Herein, the effects of cooling rate during rapid solidification of Mg40Al60 metallic glasses are studied using the molecular dynamics (MD) simulation. The system energy, pair distribution function (PDF), local structures, configuration entropy, and three-dimensional visualization are used to systematically analyze the evolution of microstructure with temperature. Results indicate that the lower the cooling rate, the lower the average atomic energy. With the decrease of cooling rate, increases the height difference between two splitting subpeaks of the second peak on PDF curves; while decreases the onset temperature of glass transition. The effect of the cooling rate on the topologically close-packed (TCP) clusters is similar to the local five-fold structures of S555: the lower the cooling rate, the more the amount of TCP clusters, and the lower the configuration entropy. In addition, most TCP clusters are centered the smaller Al atoms.

Physicochemical properties of spray dried nanoemulsions with varying final water and sugar contents

The objective of this study was to investigate the physicochemical properties of spray dried nanoemulsions having different final water and sugar contents. Formulations consisting of lactose or a 70:30 mixture of lactose: sucrose (23.9%), sodium caseinate (5.1%) and sunflower oil (11.5%) in water were heat treated (100°C, 30s), homogenized (17MPa) or microfluidized (100MPa) and spray dried at two different outlet temperatures (80 or 90°C). Nanoemulsions produced by microfluidization were more stable and less viscous than control emulsions and had lower solvent extractable free fat. Increasing dryer outlet temperature reduced water content, water activity, particle size, tapped bulk density, with a consequent increase of onset temperature of glass transition (Tg) and crystallization (Tcr) of lactose in powders. Reduction of fat globule size by microfluidization lowered Tcr of lactose, an effect attributed to the lower level of protein in the continuous phase. Partial replacement of lactose with sucrose decreased Tg and delayed crystallisation. The study demonstrated that the physical properties of powders can be altered by reducing the fat globule size of emulsions pre spray drying.

Enhanced magnetocaloric effect of a partially crystalline Gd 55Al 20Ni 25 bulk metallic glass

In this work, the magnetic properties and microstructure of a Gd 55Al 20Ni 25 bulk metallic glass (BMG) heat-treated in the supercooled liquid region are studied. The as-cast rod annealed at the onset temperature of glass transition exhibits amorphous characteristics, and the magnetic properties similar to the as-cast rod. While the partially crystalline rod prepared by heating the as-cast rod to a temperature slightly lower than the onset of crystallization temperature and then rapidly cooling to room temperature shows enhancement of the peak value of the magnetic entropy change. The enhanced magnetocaloric effect of the partially crystalline rod was anomalous because the fully crystalline ingot exhibits much lower peak values of magnetic entropy change, indicating the possible formation of some metastable intermediate phases during nucleation and growth. The existence of some metastable phases was observed by careful microstructural investigations, and their formation mechanisms and magnetic properties were studied.

Synthesis and physico‐chemical characterization of networks based on methacryloxypropyl‐grafted nano‐silica and methyl methacrylate

AbstractModel hybrid materials were prepared by using a well‐known four‐step procedure, including (a) preparation of spherical silica particles of nanometre size and narrow size‐distribution, (b) grafting of 3‐methacryloxypropyl units (MOPGS) at the surface of the particles, (c) formation of stable dispersions of the grafted particles in methyl methacrylate (MMA), and (d) copolymerization of MMA and MOPGS, initiated by UV irradiation, and then completed by thermal reaction. Progressive changes in network architecture were obtained by changing the particle diameter, d, the volume fraction of filler, ϕ, the number of MOP units grafted per surface unit of silica particle, n, and the nature of the grafting agent bearing the MOP unit. Materials of good transparency were obtained, even at high volume fractions of filler. This feature is the result of an excellent dispersion of the particles within the polymer matrix, as confirmed by transmission electron microscopy and atomic force microscopy. Swelling experiments were carried out to define the conditions, and function of d, ϕ, and n for preparing networks, in which the sol phase is almost lacking. The results were rationalized by defining a new parameter: ξD = 6ϕn/(1–ϕ)d. DSC plots were used as a first characterization of the materials. The presence of filler was accompanied by a slight increase in the glass transition onset temperature, T, and a marked broadening of the transition range, ΔTg. Both T and ΔTg were shown to increase linearly as a function of ξD. Copyright © 2004 Society of Chemical Industry

GLASS TRANSITION TEMPERATURE OF RICE KERNELS DETERMINED BY DYNAMIC MECHANICAL THERMAL ANALYSIS

A protocol was developed to measure the glass transition temperature (Tg) of brown rice kernels using dynamicmechanical thermal analysis (DMTA). In order to accommodate the sample requirements of the dynamic mechanical thermalanalyzer and prevent moisture loss during experiments, kernels were first wrapped with a thin layer of parafilm and then athin layer of aluminum foil. The wrapped kernels were then filed at both ends with fine sandpaper to produce 3 mm long ricecylinders with parallel ends. The dynamic mechanical properties of brown rice kernels were determined over a temperaturerange of -40C to 120C. During each DMTA test, the onset temperature of glass transition was measured as the Tg of a brownrice kernel, since moisture diffusion and other physical properties begin to change drastically above the onset temperature.No dramatic changes in both the storage modulus (E. ) and the loss modulus (E. ) of brown rice kernels were found aroundTg, indicating that relaxation of the amorphous phase of the rice kernel was quite limited. However, E. / E. (tan.) showeda much better transition for brown rice kernels, and thus the E. / E. vs. temperature curves were used to determine Tg valuesof rice kernels. It was found that the Tg values of rice kernels decreased linearly with increases in moisture content (MC) withinthe tested range. The Tg of rice kernels was not affected by variety.

Molecular mobility of the paracetamol amorphous form.

The purpose of this paper is to study the molecular mobility of paracetamol molecules in their amorphous state below the glass transition temperature (Tg) in order to evaluate the thermodynamic driving force which allows the amorphous form to recrystallize under different polymorphic modifications. Samples were aged at temperatures of -15, 0, 6, and 12 degrees C for periods of time from 1 h to a maximum of 360 h. The extent of physical aging was measured by a DSC study of enthalpy recovery in the glass transition region. The onset temperature of glass transition was also determined (Tg). Enthalpy recovery (deltaH) and change in heat capacity (deltaCp) were used to calculate the mean molecular relaxation time constant (tau) using the empirical Kohlausch-Williams-Watts (KWW) equation. Enthalpy recovery and onset glass transition temperature increased gradually with aging and aging temperatures. Structural equilibrium was reached experimentally only at an aging temperature of 12 degrees C (Tg-10 degrees C), according to the deltaH(infinity) results. The experimental model used is appropriate only at lower aging temperatures, while at higher ones the complexity of the system increases and molecular polymorphic arrangement could be involved. When structural equilibrium is experimentally reached, molecules can be arranged in their lowest energy state, and the polymorphic form I formation is the one preferred.

Glass transition and caking of spray‐dried lactose

Simple test methods to determine the onset temperatures of viscous flow and caking of spray‐dried amorphous lactose are described. The extent of viscous flow was measured as an increase in the density of lactose plugs within a cylindrical aluminium compaction apparatus after incubation for 3h at a specified temperature. Caking was characterized by an increase in the hardness of the lactose plugs formed. The onset temperature of viscous flow decreased with increasing water activity, Aw, and corresponded to the onset temperature of glass transition, Tg1. Glass transition temperatures were determined using both differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spin relaxation. The results suggest that elevation of the powder temperature above Tg1 promotes viscous flow and increases the potential for caking of amorphous food powders.

Dynamic Mechanical Analysis of Pultruded Glass-Graphite/Epoxy Hybrid Composites at Elevated Temperatures

In this paper the dynamic performance characteristics of pultruded glass-graphite/epoxy hybrid samples at elevated temperatures are presented. In previous research the dynamic mechanical properties of the hybrid composites have been characterized at room temperature but the wide application range of these pultruded products necessitates their characterization at high temperatures and over longer periods. The dynamic mechanical analysis technique has been employed to characterize pultruded glass-graphite/epoxy hybrid composite materials over a wide temperature range. At elevated temperatures, the rate of degradation in flexural modulus and variation in loss factor were found to be dependent on the fiber lay-up sequence. Results indicate that the width of the glass transition zone, an important design criterion, can be effectively broadened by judicious incorporation of the glass fibers. Tests repeated on post-cured pultruded products demonstrate a significant shift in the glass transition temperature towards higher side along with an increase in the onset temperature of glass transition. The frequency multiplexing technique used in conjunction with the time-temperature superposition principle proved to be an effective method for predicting the long-term behavior of these hybrids.

Scratch resistant clear coats: development of new testing methods for improved coatings

More and more car manufacturers are now demanding clear coats with improved scratch resistance from their paint suppliers. Some motor companies have developed their own tests, some have chosen a test developed by a paint supplier and others are still looking for their optimum test. The opinion prevails that there should be one test which covers all aspects of realistic damage. However, microscopic photographs and reflow experiments show that two kinds of scratches occur in reality, abrasive as well as non-abrasive, renewable ones. Different standard clear coat systems have different sensitivities to both scratch types. In the scratch tests the rating of the clear coats is divergent, because the ratio of both sorts of scratches differs. We have developed a set of two tests, where each test mainly generates one scratch type. These tests, together with physico-chemical measurements (glass transition onset temperature T g, laser optical profile scanning, indentation depths) lead to a deeper insight into scratch phenomena. One way to develop a clear coat which is resistant against all kinds of scratching damage is described.

Glass transformation phenomena in bulk and film amorphous selenium via DSC heating and cooling scans

Recently there has been an emphasis on the importance of using cooling scans in DSC experiments in studying the glass transformation kinetics of glasses. The physical interpretation of the apparent activation energy from DSC heating scans has been questioned as not being meaningful. The present paper reports glass transition temperature (Tg) measurements derived from Differential Scanning Calorimetry (DSC) experiments on bulk and film amorphous selenium samples subjected to heating, at constant rate r, and cooling, at constant rate q, scans. Film samples were prepared by thermal evaporation techniques in vacuum. It is shown that for both bulk and film forms of a–Se, within experimental errors, log(r/T2gm) vs 1/Tgm plot where Tgm is the peak glass transformation temperature, and log(r) vs 1/Tgh plot, where Tgh is the glass transition onset temperature from DSC heating scans, are parallel to the log(q) vs 1/Tgc plot where Tgc is the glass transition temperature from cooling scans. Within the Hutchinson and Kovacs formulation of the glass transformation phenomenon, the results imply that the structural contribution to the mean retardation time, τ, is negligible in comparison with the temperature dependent part. The mean structural relaxation time for both bulk and film forms was found to exhibit a typical Vogel-Tammann-Fulcher type of temperature dependence. Furthermore, the structural relaxation rate was observed to be inversely proportional to the viscosity, η(T), implying that the mean structural retardation time is proportional to the viscosity, τ ∼ η. The results also confirm that the earlier studies of glass transformation kinetics in a–Se utilizing only DSC heating scans remain meaningful.

The glass transition in novolac phenolic molding compounds and the kinetics of its development during cure and postcure. II

AbstractThe change in onset temperature of the glass transition in a novolac phenolic molding compound was related to the time and temperature of cure. A mathematical model consisting of a single function of time and temperature fit glass transition onset temperature ranging from 87 to 242°C. These glass transition temperatures had developed during molding at temperatures from 132 to 218°C at times of 10–1800 s. The same model predicted the glass transition and onset temperature of samples postcured to a glass transition temperature of 287°C. The mathematical model resulted from a kinetic rate law treatment. This treatment, however, did not include a secondary reaction which causes a small exotherm above 210°C in DSC testing. The resulting rate law still appears to provide a useful approximation of the development of the Tg and the cure rate in novolac phenolic molding compounds over practical molding and postcure conditions.